An automatic model-based controller is used for stabilization of gas lifted oil wells. The controller stabilizes the pressures, temperatures and flow rates in the well through active feedback control and continuous manipulation of the opening of the production choke and/or the opening of the gas injection choke as a dynamic function of available process measurements.
The system comprises a production choke for control of the product from the oil well and/or a gas injection choke for control of lift-gas to the annulus.
Unstable production conditions often occur for oil wells where gas lift is used to increase the oil production. This is a serious problem due to the fact that the instabilities often occur in the most optimal area of operation for the gas lifted wells, thus causing the well to produce less than the system design capacity.
Unstable production of gas lifted wells is not in agreement with smooth operation and it implies safety aspects and shutdown risks. The total oil and gas production must usually be less than the systems design capacity (e.g. of the separators) to allow for the peak production. Unstable operation decreases sharply the lift gas efficiency, and leads to difficulties with gas lift allocation computation.
Unstable production of gas lifted wells may be caused by a large variety of factors, such as incorrect gas lift string design, improper valve setting, wrongly sized injection valve port, variation in supply pressure, or valve leaking or plugging. It is often difficult to find the origin of the instabilities. As a result, a pragmatic approach has often been used to solve the problem of unstable production in short term. For example, if unstable production occurs, the operator often increases the amount of lift gas or increases the back-pressure by adjusting the wellhead production choke to a smaller opening (choking). Although these methods can be effective in reducing the instabilities, the production is still inefficient as either too much lift gas is used (high cost and limited availability of lift gas) or the well is producing against a high back pressure (at low rate). In most cases, too much gas is injected into the gas lifted wells or the production rate is not maximized.
The present invention relates to a method and a stabilizing well controller for stabilization of gas lifted oil wells without using lift-gas in an inefficient way or by introducing a high static back pressure. The characterizing features of the method are given in claim 1. The characterizing features of the stabilizing well controller are given in claim 14.
This concept for a model-based stabilizing gas lift controller for automatic and on-line control represents a number of inventive steps. First and foremost, our concept is able to stabilize the pressures, temperatures and flow rates of a gas lifted well in an operating point that is unstable in open-loop (i.e., when no active control is used). The unstable production phenomena for a gas lifted well is eliminated, without increasing the mean gas lift injection rate, through active and continuous manipulation of the opening of the production choke and/or the opening of the gas injection choke as a dynamic function of available process measurements. The model-based stabilizing gas lifted well controller provides a way to stabilize gas lifted wells with different measurement devices (sensors) available for control purposes.
The model based stabilizing gas lifted well controller is also characterized in that it allows that the control error, which is a function of an externally given (optimal) reference operating point and the real operating point, at any time may be minimized with respect to a predefined model-based (integral) norm.